1. Field of the Invention
The invention relates to an interconnect structure in an integrated circuit, and more particularly to a method of improving electromigration (EM) resistance by modifying the interconnect structure.
2. Description of the Related Art
Integrated circuits are made of devices, such as field effect transistors (FETs) and bipolar devices, formed in and on a semiconductor substrate, and the multilevel interconnect structures that form connections to and between the various devices in and on the semiconductor substrate. Many integrated circuits include closely spaced arrays of devices that are accessed by and connected to one or more arrays for parallel wiring lines formed above the substrate and the devices. Arrays of closely spaced wiring lines are a familiar feature of integrated circuit memories because of the regularity of these circuits. Such memories include nonvolatile memories like ROM, EEPROM and flash EEPROM, dynamic random access memories (DRAM) and static random access memories. Because of the requirements of routing interconnects efficiently while using as few layers of interconnects as possible, parallel arrays of wiring lines are also featured in digital signal processors, microprocessors and even more random sorts of logic circuits.
Conductive lines are very often coupled with the silicon diffusion regions or other conductive lines by a contact/via. As mentioned above, because of the requirement that there be as few layers of multilevel interconnects as possible, the contacts/vias are in a form of an N.times.M via/contact array. Considering an even distribution condition, each contact/via carries a current flow of 1/N.times.M of the total input current. However, in practical use, due to the different conductive paths, the equivalent resistance of each contact/via is different, so that the current flowing through each contact/via is different. As a consequence, the contact/via having a larger equivalent resistance received a smaller current, and the contact/via having a smaller equivalent resistance receives a larger current. For the contact/via receiving a larger current, the EM lifetime is shorter.
Normally, the contacts/vias are designed perpendicular to the conductive lines. In FIG. 1, a conventional vertical contact/via array is shown. The current flows into conductive line M1 through the contact/via A.sub.11, A.sub.12, A.sub.13 and A.sub.14 to the conductive line M2.
In FIG. 2, an equivalent circuit of the structure shown in FIG. 1 is shown. Assume that a resistor r exists between each contact/via A.sub.11, A.sub.12, A.sub.13 and A.sub.14, and the resistance of each contact/via A.sub.11, A.sub.12, A.sub.13 and A.sub.14 is r.sub.c. When the total current flowing into conductive line M1 is I, the relationship between the currents I.sub.1, I.sub.2, I.sub.3 and I.sub.4 respectively flowing through each contact via A.sub.11, A.sub.12, A.sub.13 and A.sub.14 is represented as: ##EQU1## Assuming that r/r.sub.c =0.05, I.sub.1 /I.sub.4 =1.31, I.sub.2 /I.sub.4 =1.15, and I.sub.3 /I.sub.4 =1.05. The current carried by the contact/via A.sub.11 is 1.31 times the current carried by the contact/via A.sub.14. According to Black's Equation, the electromigration lifetime is inversely proportional to the square of the current carried by a contact/via. Therefore, the EM lifetime of the contact/via A.sub.11 is only (1/1.31).sup.2 .apprxeq.0.58 times the EM lifetime of the contact/via A.sub.14.
In FIG. 3, a 4.times.4 vertical contact/via array is shown. A total current flows from the conductive lines M1 through the contact/via array composed of A.sub.11, A.sub.12, . . . , A.sub.44 to the conductive lines M2. Similarly, each of the contact/via A.sub.11, A.sub.12, . . . , A.sub.44 carries a current in accordance with the relation shown above. After normalization, the current I.sub.11, I.sub.12, . . . , I.sub.44 carried by each contact/via A.sub.11, A.sub.12, . . . , A.sub.44 in the contact/via array is: ##EQU2## As shown above, the ratio of current carried by the contact/via A.sub.44 with a longest conductive path to the contact/via A.sub.11 with a shortest conductive path is about 0.79/1.39.apprxeq.1/1.76. Consequently, in comparison with an even distribution condition, the EM lifetime is only about (1/1.35).sup.2 .apprxeq.55%.